Thank you.
It's totally fun for me to come and interact with the vibrating energy that you young people bring.
It's the reason that people go into teaching.
My wife and I have been in that racket now for 80 something years aggregated, it's totally good fun.
So in Boulder it's not a bad place to have your labs.
And the topic which we're usually teaching students now, are primary school students,
because we're a little bit afraid about whether there is going to be your replacements in 5 years or 10 years.
So in their case we're talking about how to learn.
In your case the topic I think is more developing a scientific taste, to think about what would be interesting to learn.
And I'm just going to try to use some examples.
As you know lasers had their 50th celebration last year.
And that's made possible precision spectroscopy.
I worked for the government agency that's concerned with mass, length and time.
So we've has some fun with that.
Spectroscopy has improved by 10 orders of magnitude in resolution.
I promised that I would retire when we got to 1 megahertz and then we passed that and came to 1 kilohertz and now 1 hertz and then something in millihertz.
So, there's a lot of good things to try.
So the kind of questions that kind of bother me when I go for a walk is: Ok, so we have atoms and physicists talk about their being identical.
Well, that's confusing to me because I think identical is an operator that doesn't mean comparing 2 things.
To me it means a thing.
But equivalent maybe is better.
Then the other issue is: How narrow can the laser be?
Most of the things that make the laser have a broad spectrum are associated with fluctuations in the mechanics of it.
Maybe we can make that better, cryogenics or some other idea.
What about the fundamental constants?
Are those, for example, the mass of the electron or the ratio of proton masses to electron masses?
We can build clocks that depend on different things.
Internal energies for example, atoms and so on.
And one of the kind of clocks that I'm working to make happen now is one that depends on the vibration of molecules.
Because you recognise that the mass of some nucleus depends on the strong force that's glued together and we can change the isotopes that we have and that changes the mass by a tiny bit.
And part of the part which is not the mass of the parts is strong force.
So we would have a window with some reduction of sensitivity.
But a window to see how these things work.
So that would be interesting.
Now, not all places have the same values.
Here's a town 30 miles from us which offers you a free Nobel Prize if you buy something in their store.
They sell guns, ammo, Glock and even safes that you can carry.
Well, a few centuries before there were thoughtful people and they came to consider the issue about atoms.
I guess it's because you're grinding this beautiful stone and what is the smallest particle that can be in it.
Atoms have been in our universe as physicists of course for some time.
Robbie was perhaps the person that really got serious about trying to find out what their properties were.
He suggested that you could have an atomic beam with electromagnetic fields to interact with it.
And a resonance demonstration was made in 1955.
So the idea is that once you measure frequency, that's the way to get more data.
And then laser jocks like me have come along later and just extended this to higher and higher frequencies and the new magic understanding is lattice traps.
And one is supposed to measure the phase that's going along, write down the phase at different times for example regularly scheduled times.
And you find that the averaging is more attractive.
So you can obtain more information per second than was previously obtained.
So, I'd like to just say quickly about these accurate atomic clocks.
The ones on the left, Kurt was having trouble with this.
Here was the demonstration level and the second was chosen in 1960 to be not a spin of the earth or even the earth in orbit but rather some number of oscillations of caesium atoms.
And then one could remove some of the technical difficulties.
In middle 1990's the idea of using lasers for atomic cooling came up and so then some new things happened.
It takes a while for that to have an impact.
So the fountain clock was about on schedule.
We tried to build optical clocks.
In the beginning we had technical problems and concept problems.
But finally the combination of cooling and better laser stuff, we came surging down also.
So, here is where the NIST fountain is now.
Actually a little bit below 10^-16.
Here is what has been achieved by thinking of all the mistakes that might be in determining the frequency of mercury+.
And here is where we are with the strontium atom in an optical lattice.
I'll come back to this in more detail in a second.
Anyway, the numbers on the right are the ones to look at, it's 10^-16.
So we have 2 ways of getting things that are fun to think about.
One is to go to where they are.
You can see where the mountains are and you build an accelerator which can excite that energy.
You can have that mass and it's in your laboratory for a femtosecond or exasecond or some other small time.
Another is to see that there is something, a mountain range that's away because the ground is tilted where we are.
And if we measure dispersion, we have a 1 over the detuning dependence.
So these 16 or 18 digits really are available to us for exploring what the phenomenon is.
So, strontium is really nice when it actually holds the record for the quality factor and the signal noise.
If we go from frequency to length, in the Treaty of the Metre it was decided to use bars like this of special alloys.
But of course lasers came and it was possible to get sub-Doppler peaks which were really narrow and very strong.
So you can have high signal noise ratio.
So if you make wavelength measurement compared with the standard, these are the laser fringes, these are the fringes of the standard, it's quite clear which way you'd like to go.
Even so with a krypton standard you could have something in 10^-9.
So in the end we're going to be wanting to measure optical frequencies.
We're going to start at this caesium frequency, 9.192.631.770.
It used to be fun to dial that number.
Now it shows up, it says not working number.
But anyway the place where you'd like to go might be this mercury ion transition.
It's higher in frequency.
It's a petahertz roughly in the UV.
So the frequency ratio we need is 115,000.
So now we need to talk about how are we going to do this.
115,000 and we need to keep track of that in the 10^-16 domain.
So it's not going to be done with some analogue method to take the protractor and lay down this thing.
We've got to have something that's got gears in it, optical gears.
So here's what was implemented in Boulder.
Ken Evenson was interested in these lasers that made all kinds of different colours with molecular transitions.
Joe Wells, his colleague, noticed that this was 8 metres long.
And that's what you need in order to get enough optical power that you can drive a second harmonic kind of process and get a factor of 2.
And so then you're thinking: "Wow, factor 2 is not very much because I've got 115,000 to do.
Now what's the bits representation factor of 2?
To that well it's...
You get 64k for 16 bits so this is 18 bits or something like that.
So it's 18 lasers or 15 lasers depending on how ambitious you are which are going to have to be all coupled together."
So, what happened was that we did that and this metre bar that I showed you was already replaced by a wavelength.
Michelson noticed that that was a good thing to do, 1.6 million waves.
We as an international collaboration, all the guys like me from different standards labs, couldn't agree whose laser we should use for the length standard
but we could all agree that the speed of light was fundamental.
And after some negotiating with 46 countries sitting around a table, 2 guys from each country, we could finally agree about this.
But it turned out that every country got 1 vote and that didn't respect the aero bars.
So we had these really small aero bars, so this number should have been a slightly different one.
But only 2 or 3 labs could measure it so it really doesn't matter.
So anyway, there's going to be a quiz as you go out.
So this represents really a culture change
because we have a system of units that are derived in a way that you can go into your lab in Switzerland or your lab in China or any other place and realise the fundamental standards there.
So your experiment can be expressed in terms of those standard units.
And then a comparison can be done by you printing it in a journal and someone reads the journal.
Otherwise you have to transport a standard from one place to another.
So these standards that are connected to atomic physics are the key for us having really the scientific society that we have now.
Ok and 2 centuries before you could do science and the question would be when you pour these together does it turn blue or yellow but we're past that.
Now we're really trying to do numbers for the science.
Well, the thing which has been fun in the last decade, even a little bit more for Professor Hänsch's group and ours, is this optical frequency combs.
So we have this nice building in Boulder.
The program has lasted a long time.
I mentioned it's 50 years of lasers, it's also 50 years of me playing with lasers.
And in the meantime a brilliant young colleague has come to the fore and is now running the laboratory beautifully.
Over that time the people of America and their parents and their children have paid in some taxes and run these agencies that then have given funds to us.
One thing which we can contribute back is some knowledge.
Well, we contributed 3 kids into the gene pool, we even have grandchildren.
But in this building we have started with 8 experimental people and now we have this JILA institute, perhaps some of you have heard of the name.
And in these 50 years we've worked hard on trying to understand how does one work cooperatively to make research be the most powerful.
So, let me just give you a top view of what happened.
Some of you started in this mid time, a few of us older folks that have trouble with knees or hearts or whatever started back here and we watched the laser come on.
I didn't pay much attention to this gas laser because that was announced 6 days after Lindy and I had our first son.
So I was otherwise interested.
But the bottom here is about pulse lasers and the top is about continuously running lasers.
And initially it seemed like these were totally different.
What's a pulse laser for?
It's to generate a lot of heat.
For example, you can burn razor blades and a career goal might be to burn through 137 razor blades.
Well, that's not my taste.
I would rather go some place up here where there's frequencies to measure, the stabiliser I just told you about.
And the fun place is over here in the last decade.
So by the time we got to 1997 these frequency stabilised lasers had really increased.
This is the wavelength axis or frequency and terahertz axis on the top.
Lots of different systems, this is by far not all.
These are just the ones whose wavelengths had been measured very precisely.
And the literature carried a few important things, some kind of things under the radar.
The Hänsch paper in '78, Chebotyef paper in the same epoch.
And then a little bit more expansive paper just before the new millennium.
And they were proposing this absurd idea that you could measure something sharp in frequency by having something sharp in time.
And your training probably has some cup on it like mine that these are conjugate parameters.
And if you have a sharp frequency you can't have a sharp time.
But that's just because we haven't thought well enough because if you have many sharp pulses in time there is a clock in it.
It's the repetition frequency, how many radians of optical frequency go by during these time intervals.
And that will show up as the spacing of the comb.
Each one of these is so narrow in time that its Fourier domain includes frequencies which are clear up here in the optical range.
So that's how one gets out of this little intellectual puzzle.
So, the Greeks liked the concept of atoms.
The people in Columbia with Robby liked the concept of atoms.
I like atoms that have optical absorption.
So for optical cooling these kind of atoms with simple spectroscopy...
Well, actually there's a little bit of complexity because electrons have spin of a half and so these are doublets.
But there's another category of atoms which are in group 2 that I really like.
Introduced, as any good idea is in this world, simultaneously by lots of different groups and the concept is originally that you have a strong transition.
This is in a system which 2 electrons have coupled together to have a spin of a ½ + ½ to give 0.
So this has simple energy structure, strong transitions and that means that optical forces can slow this kind of atom down.
But to build my clocks out of, my dream was to use this magnetic transition
which had something in milliseconds time domain that corresponds to 60 hertz or 100 hertz or kilohertz or something of that scale.
And the younger people saw that there was a much more intelligent thing and that's to use the fact that in the triplet system we have the nucleus spins added up to have S=1.
And so we have these levels.
And in particular this J=0, you can remember from trying to do Clebsch-Gordan algebra that there's no matrix element for that.
And that's true at very high precision.
So the only way that you can have a transition like this is some kind of perturbation.
A hyperfine perturbation for example gives 100 seconds.
Now 100 seconds, let's see how many cycles go by if our frequency is 5 times 10^14.
Wow that's a lot of cycles.
We only have to split that to just a little bit and we're going to really have some precision.
So, let me just sketch to you what happened in this comb time.
Lots of different ideas came together and what really happened was...
The way that I showed up in this is that I just like to learn a lot of things.
I heard a talk about some magic fibre and that made it possible to follow the idea professor Hänsch had published.
And in a collaboration between Hänsch group and our group we ended up getting this system to work in one step.
To connect from the optics and the radio frequency domain.
That was in early 2000; by 2001 there was 20 papers from the standards laboratories.
And 4 or 5 years later there was a Nobel Prize.
So other people thought that that was important.
It was certainly important to me because I had spent all the years, my professional years before that trying to learn how to measure frequency and it needed 15 people at the maximum.
And now here is something where you can just plug into the side and go have a coffee and you get the frequency.
It's due to the magic of this fibre.
The outside diameter of this is the same as these fibres that Doctor Wüthrich talked about.
We carry around in the top 100 microns or so.
Inside this fibre is a solid core and then some air bubbles around it.
This was really a solid rod and air tubes around.
Then put a shell around it and then it's this big.
And you give it to the guys with the carbon dioxide laser and they heat it and they stretch it.
And so then they give you back a spool of 20 kilometres of fibre.
That's this mass that's drawn out to that length.
So this gets smaller and this is only now 1.8 microns in diameter.
If you put strong laser in that, that's really going to shake those atoms because the electric field is like a third of the electric field that holds atoms and molecules together.
And a red light comes in at the top and you see these coloured things generating.
Now the speed of light is a big number, so this is all lit up at the same time almost.
And I went to a post deadline paper in a quantum electronics meeting and saw this.
So I knew this was pulsing at 100 million times a second because the laser was.
But all these colours were simultaneously present.
So this is really some kind of optical comb that's starting.
This optical comb has the character then in the time domain, it looks like pulses.
But we've magnified the time out and we've got breaks in the axis.
So in the pulses you can even see a little bit about the optical wiggles that are there.
Some interesting details about this in the optical range if you measure frequency.
They're spread out nicely.
We have a self locking method that was invented by, I don't know, 14 people or something and published again simultaneously.
So here is this fibre.
So converting 15 people in that lab that I just showed you that had so many long lasers in it.
The spectrum that we put in to it from the Ti sapphire laser is broadened out by this.
So it can then form an octave of bandwidth, factor 2 of bandwidth.
So this is something like a magic rainbow fibre.
And when you take all the magic away and just look at what's in the hardware.
Here is what replaced all those lasers.
It's just a way to turn one laser colour into a million laser colours.
That's beginning this complete revolution in how one is going to do optical frequency spectroscopy.
So I'd like to speed up here and summarise what's happened.
An independent community took those pulse lasers and they wanted them to pulse faster so they started playing with femtosecond lasers.
They'd get narrow pulses but they could have faster and faster rates.
And finally 100 megahertz or something.
The people in Boulder, in Novosibirsk and other places tried to make lasers more stable.
Finally you can talk about the phase of this optical frequency and even talk about the milliradiance of phase of that.
And some other community is interested in just the total breakdown of any kind of linear approximations.
So the order of non-linearity, the scale is 1 million.
So that's not a small signal expansion.
So this led with these 3 or 4 ideas to some kind of new concept, this optical comb, 39 years after we had lasers.
And it's developed in collaboration with the Garching and Boulder groups.
So what's happened then is that this barrier that's above this is continues lasers and below this is pulse lasers.
It begins to get fuzzy here and finally is completely gone.
So now if you have an optical comb you can decide: "Ok, I'm going to talk about this as these femtosecond laser pulses in time and they have this rate."
Or you could equally well come to it with a measuring apparatus that says: "I want to know what frequencies are in this."
And you have to resolve down and find which one of the comb lines you're talking about.
And then you find out that this is maybe of the order of 1 million lasers and they can be sub-hertz in their line width.
So this barrier conceptually between pulse and CW is completely gone.
So, I'd just like to say one thing which is obvious after you see it but it's important to have in your mind.
What I've talked about is the comb that's in frequency.
What I've talked to you about is the source of pulses in time.
So here are a few of the wave forms of those that would be added up.
So here's 5 of these and if I re-scale it so that I'm adding up some place where they all are in phase, well, it's taller.
In fact if I have N of these waves the amplitude is increased by N and the power is by N^2.
And since N is about a million waves that are added coherently together, N^2 begins to be an interesting thing.
And that's how you go from a little power that's not much different power than what this flash light gives to the kind where it's shredding atoms, can shred atoms.
Ok, if you get just a little less than that it jiggles the atoms so much that they make this enormous non-linearity.
Of course, the exchange for that is that the duty factor is short.
So you have N^2 of power but it's only on for 1 Nth.
The more frequencies you add the more narrow that becomes.
So it's still just average power which is N times bigger than the component power.
That's what you expect.
So if we zoom in on this what you see is this green pointer, this one might not be completely frequency stable but I have one I the lab which really is.
And you can talk about its frequency and you can know its frequency for a second in advance.
And it has a time scale which is rather short.
It's 15 decimals and 2 femtoseconds per cycle.
But if we look on a different time scale, this green light that you can see oscillating here, it's plus sometimes and it's minus sometimes.
Well, it's basically on this slower scale, it's always on.
That's how this looks like and you don't see that this is an oscillating signal.
But we do have instrumentation in the lab with this faster telescope that we could see that the laser is emitting pulses with for example 1 nanosecond separation.
So that's fine, now let's try to compare these.
I told you that it's possible with that fibre to make this 1 nanosecond pulse have energy that extended up to the optical frequency range.
And so it looks a bit this way.
It's chirped because in fact the field is so much that the index refraction depends on what the field is.
So the component colours seem like they're fully blurred out.
But it's completely the same on each flash.
So there is some phase relationship between this and between another one and you can see here that it happens that the envelop and these carrier oscillations...
I think we've got a little problem with this pointer...
But anyway they're in alignment and so what you can learn from this is that there's a synchronisation in place.
So the optical frequency is synchronised with these pulses, that you have the same shape each time.
So, I know that there's a harmonic relationship between them.
So this is just magic because you start out with no knowledge at all except the early digits.
And you find ok there's an energy relationship.
Now I change this repetition rate by 1 part in a million and I change the integer by 1 and then I can catch that.
Do this 3 times or so and then you're certain of what the harmonic is.
It's 1 million and it's 03 at the end and then it changes 04 or 02.
So it's unambiguous.
And it's just...
Magic is too strong, but it's really a marvellous kind of tool.
So, let me just insist again that you walk away with the right idea.
If you have repetitive pulse trains, you're going to get a comb in time and a comb in space.
And the frequency width of a comb line if you start with pulses; it depends on how long you measure it.
And if you want to measure it in the conjugate range, how wide it is in time, depends on what's the band width of the ones that you've gotten together all coherent.
So this is really an enhancement of our understanding.
And I was taught about Fourier things in an epoch when this idea of using plenty of pulses hadn't come yet and we're indebted to these gentlemen for a jump ahead.
So what I've told you about is a tool.
It's just as good as that one.
It's some kind of gears that connects microwaves and optics.
There is no...
There may be a little bit of chatter in this but there's no slipped pulses.
So there's positive interference.
Well it's made a fantastically good time.
We've had a few joys at research, Munich research.
Outdoor laboratory as you saw there.
Some paper for one to discuss with your parents about or you can find some primary literature there.
And so now I'll tell you in the end just a couple of things about an old person and his career.
Well, that was my career and there it goes.
And Jun Ye came on board and in the 10 years since he's been there all these kind of things have been possible using the comb.
So partly it's he is way smart and partly it's that this is a really explosion time.
Who would believe 50 years after we have lasers that the growth rate has increased.
There's just fantastic possibilities.
So don't sell your stock yet, it's going to be good.
Here is one thing which is interesting.
How do you make the laser be stable?
Well the reason it wasn't stable is mechanics were changing.
But if you mount it in the middle and you hold it vertically then part is compressed by the gravity and part is stretched by the gravity and so this more or less cancels the gravity out.
So you can see ultimately narrow lines.
And to prove that it's narrow you can send it from our labs in the university over to the NIST labs.
We have 2 different colours so we have to use combs to compare them.
But you get a hertz beat and there's no way of doing.
Who cares?
Well, people that build accelerators care, people that have to synchronise all these klystrons.
People that try to build millimetre wave telescopes in Chile really care.
It's so impressive how quickly they pick this up.
It's come to my door really soon.
And this gang of 20 people on the earth that are interested in super-precise clocks also care.
So you can transmit a 10^-7 frequency knowledge in one second.
It's just amazing.
So, where one would like to go then is to look at atoms with a higher resolution.
So the ideal trap for this atom that I showed you.
There's going to be interactions of this level up and this level up.
And the magic that's happened due to Katori and due to Kimble is that there might be a colour where the interactions are the same.
I kind of make that explicit, that some second order Stark interaction.
So, as you are already aware this is my usual 1 hour talk which is being given to you at warp speed.
But it is possible with this comb business then to simple scan.
So there's no averaging, this is just turn a frequency to this value and this is in hertz, 1 hertz per 1 box.
So you change the optical frequency by half a hertz and measure what's the transition probability.
Change the optical frequency by another half a hertz, measure what's the...
just, yeah, here it is.
So I am not the person to announce the results but if you keep watching in the literature you'll find spectacular advances come out of Jun's group.
One more minute, ok.
So, one thing which is interesting about molecules and old people is that we would like some diagnostics.
Here's a student that looks green and maybe has a drinking problem or something but you can see the spectrum of what's in him.
You can see that the body is trying to get this different isotopes out.
You can see that it's culturally hard to get this multidisciplinary research funded.
So there's a lot of things which I should tell you about in the longer time but you can build a nice instrument.
What I would like to conclude with, this gives me the chance, is some people who have actually made this work happen.
Now, how do I go to talk about the people that made it happen?
So, this is again multicultural, we have some Asian faces and some young faces and it's pretty fun.
We have some young faces in the school.
Lindy and I are bringing some joy into these kids.
They come from families where the idea that kids and play would be...
These girls were totally fascinated by this complicated gear system.
Is it going to turn right hand or left hand?
And there's some books that are available if you're a parent, there's really a fantastic time.
So I thank you for your attention and here is a few references.
Applause.